If you need a new liver, doctors have about twelve hours to transport it from a donor. That ticking clock severely limits the ability of doctors to get organs to patients.

Now researchers have demonstrated a method that kept rat livers viable up to four days.

The scientists lowered the livers to below freezing temperatures, while flooding the tissue with antifreeze chemicals to prevent the formation of damaging ice crystals.

But such cooling alone is not sufficient, due in part to the liver’s wide variety of cell types and functions. So the researchers also used machine perfusion: as the livers were cooled they were flushed with solutions that kept them operational. They were perfused again as they were brought back to above-freezing temps.